Polishing removes marks and traces of tools or moulds from the surface of the crystal. It is carried out on grinding wheels of different diameters. In the 17th century, in Bohemia, glassmakers developed a wheel engraving technique inspired by the world of gemstone engravers. Today, this technique has been modernised and uses copper or lead wheels fixed to an electric lathe. When the wheel comes into contact with an abrasive powder (carborundum), the decoration is formed on the surface of the crystal. The operation is carried out under a trickle of water which cools the crystal, overheated by the friction, and prevents it from bursting.